Hydraulic pulse generator

ABSTRACT

A device for creating a flow of liquid and for imparting a pulsating pressure condition thereto, such as, for example, a device for pumping liquid fuel to the nozzle of an injection type carburetor.

United States Patent Backman et all. Mar. 5, 1974 HYDRAULIC PULSE GENERATOR [56] Refierences Cifited [75] Inventors: Stun-e Anders Backman; Knufl UNITED STATES PATENTS Lllflvig Winquisi, both of Orebro, 2,914,240 11 1959 Vorech 417 552 Sweden 2,442,631 6/1948 Winkler.... 417/443 1731 Assignee: Mm fi Benicasim, i233??? 551332 ;2,l;i".?f5;... 2151i}; Castenon, Spam 1,218,903 3 1917 Scovel, 1L 417/552 [22] Filed 1, 1971 FOREIGN PATENTS OR APPLKCATIONS [21] A N0.: 203,922 923,589 12 1954 Germany 417/570 Related US. Application Data P E W L F h rzmary xammer 1 1am ree [62] 32:??? March 1969 Attorney, Agent, or Firm-Stevens, Davis, Miller &

Mosher [30] Foreign Appiicatien Priority mm Aug. 23, 1968 Sweden 11405/68 s C Dec. 6, 1968 Sweden 16743/68 A d i f ting 3 H of liquid and for imparting a pulsating pressure condition thereto, such as, for [52] US. CB. 417/274, 417/471 example a device for pumping liquid fuel to the 511 11 1. C1 F041 19/00 Z16 of an injection type carburetot [58] Fneld of Search 417/552, 471, 443, 274

16 Claims, 2 Drawing Figures V HYDRAULIC PULSE GENERATOR This is a division of application 809,685, filed Mar. 24, l969, now U.S. Pat. No. 3,645,648.

The present invention relates to a hydraulic pulse generator, i.e., a device which from an undefined flow of liquid creates a flow of pulsating liquid. One requirement placed upon such pulsating liquid flows is that the flow comprises distinct and well defined pulses of the desired magnitude.

Pulse generators known heretofore have not fulfilled this requirement to the extent desired, especially when concerned with volatile liquids. When volatile liquids flow through valves, etc. small bubbles of gas appear in the liquid rendering the liquid highly compressible and to varying degrees. This variable compressibility makes it impossible to obtain well defined pulses of the desired magnitude. Another disadvantage associated with previously known pulse generators is their inability to adapt themselves to varying flowrequirements. Known pulse generators usually function at a constant full flow output with a non-consumed portion of the flow recirculating via a by-pass line provided with an overflow valve, a method which is both uneconomical and which affects the distinctiveness of the pulses deleteriously.

One field of use in which the aforementioned problems are relevant concerns a certain type of injection carburetor for internal combustion engines, where engine fuel is supplied to an injection nozzle at a pulsating pressure, i.e., in the form of pulses, to initiate and main tain vibratory movements of a valve body. A more detailed description of such a carburetor is given in U.S. Pat. application, Ser. No. 536,550, Swedish patent No. 305,980 and U.S. Pat. application No. 9179/68, these carburetors being of the type to which the present invention is applicable. It is important in these carburetors that the fuel be pumped to the injection nozzle in well defined and distinct pulses. The problem, however, is that petrol, for instance, is a very volatile liquid and contains certain quantities of gaseous hydrocarbon in solution. This means that the risk of gas bubbles appearing in the petrol is high, especially given the pressure drops to which the liquid is subjected while passing through the narrow passages existing in known pressure and suction valves, particularly when such valves are spring loaded. These gas bubbles render the liquid compressible to varying degrees, thereby deleteriously affecting the functioning of the pulse generator and thereby the functioning of the carburetor.

A further disadvantage in known pulse generators resides in their being incapable to immediately respond to varying flow requirements, such as, varying fuel requirements of an internal combustion engine. An automobile engine, for instance, operates at maximum capacity (corresponding to maximum fuel flow requirement) only during a small proportion of its total operating span. Consequently, it is particularly uneconomical to employ a pulse generator which functions at a constant maximum capacity in conjunction with a carburetor. In this regard, the power required to drive the pulse generator itself constitutes a significant power loss.

It is, therefore, an object of this invention to provide a novel pulse generator for liquids which overcomes the disadvantages of known pulse generators, with particular regard to the aforementioned disadvantages which, however, are not limitative'of the scope of this invention.

It is a specific object to provide an improved pulse generator for pumping liquid fuels to a carburetor of the fuel injection type.

It is a further specific object to provide a pulse generator which is especially adapted to pump a liquid in accordance with distinct, well defined pulses of a particular magnitude which itself may be varied to suit different requirements.

It is a further specific object to provide a pulse generator which itself consumes a minimum of power.

It is a further specific object to provide a pulse generator for the carburetor of an internal combustion engine and which generator may be driven by an electric motor drawing power from the electrical system associated with said engine.

Other objects are those which are inherent in the herein presented disclosure of two preferred embodiments of realization of the invention, a detailed description of which follows with reference to the accompanying drawing wherein:

FIG. 1 is a sectional view ofa pulse generator according to a first embodiment of this invention; and,

FIG. 2 is a sectional view of a pulse generator according to a second embodiment of this invention.

The objects of this invention are realized generally through an arrangement comprising a driven reciprocable piston means which is constructed to function as a non-return valve for liquid being pumped thereby, said piston means being interconnected with a drive means therefor through an axially resilient coupling.

A first embodiment of the hydraulic pulse generator is shown in FIG. 1 of the drawing and includes a housing 1 providing a cylindrical space 2 in which a pulse piston 3 is reciprocably displaceably arranged. The piston 3 includes longitudinal or axially extending through passages 4 and an annular recess 5 defining an inlet port 6. Leading to the inlet port 6 is a feed port 7. Mounted in a central bottom opening 8 in the piston is a valve body 9. The valve body, which comprises a valve disk portion 10 and a stern portion 11, constitutes together with a valve seat 12 on the piston a non-return valve and is, for this purpose, movable relative to the piston along its direction of axial movement. The axial displaceability of the valve body results from the fact that it is secured to the piston 3 by means of a pin 14 which is fixedly secured to the piston and which extends through transverse opening ll3 in the stem portion 11 of said valve body, there being a certain degree of clearance between the outer periphery of the pin 14 and the wall defining opening 13, this clearance being exaggerated in the drawing for purposes of illustration only. Valve body 9, therefore, is capable of being displaced axially away from piston 3 to an extent equal to said clearance, such displacement resulting in an annular passage being formed between the outer periphery of valve body 9 and seat 12.

The axially endmost radial surfaceof valve body 9 constitutes the working end of the piston 3 and serves to delimit the pulse or compression chamber 15 from the remainder of cylindrical space 2. An outlet passage 16 leads from pulse chamber 15 to externally of housing 1.

Piston 3 comprises a piston rod means generally denoted 17 which in turn is made up of two axially successive rod portions 18 and 19 which are axially coupled together by means of a resilient coupling 20.

Coupling comprises a sleeve 21 in whose bore rod portion 19 is guidingly slidably arranged, sleeve 21 in turn being guidingly slidably arranged within a bore of housing 1. Rod portion 19 comprises a pair of longitudinally spaced apart abutment members 23 and 24, the first of which is abuttable against one end of the bore in sleeve 21 and the second of which, 24, serves to confine one end of a compression type spring such as cup spring means 22 whose other end abuts against a shoulder 25 of sleeve 21.

The other rod portion 18 is integral at one end thereof with piston 3 and includes an abutment flange 36 at its other end which confines one end of compression spring whose other end abuts against shoulder 37 of the housing 1. The extreme outer end of rod portion 18 abuts against a flat solid disk 27 which rests against shoulder 28 of sleeve 21. Rod portion 18 is guidingly slidably fitted within bore 29 in housing 1.

Abutment member 24 is axially adjustable along the length of rod portion 19 by means of threaded nut means 26 in order to vary the compressive force in spring means 22.

The external or free end of rod portion 19 is provided with a follower shoe 31 which in turn cooperates with a drive means to impart a periodic axial thrust to the rod portion 19. Such a drive means comprises a motor 34 on whose shaft (which runs perpendicular to the reciprocation axis of piston 3) is mounted an eccentric or cam 33 which bears against the surface of shoe 31. The axial position of rod portion 19 relative to shoe 31 can be varied through threaded means 32 in order to vary the length of stroke of the piston 3. The speed of motor 34 determines the frequency with which rod portion 19 is driven by follower 33 and, consequently, the frequency of the liquid pulses discharged from chamber 15 into passage 16.

In summary, therefore, it is seen that piston 3 is normally urged, with reference to the drawing figure, in a downward driection by compression spring 35, and in turn said piston acting through rod portion 18 urges sleeve 21 downwardly which, acting through spring 22, urges rod portion 29 downwardly. The two rod portions 18 and 19 are, therefore, resiliently interconnected in that they may be displaced in either axial direction relative to each other pursuant to a corresponding flexing of the spring means 35 or 22.

The outlet or discharge passage 16 can lead to an injector nozzle 39 ofa fuel injection type carburetor such as disclosed in the aforementioned patent applications. For the purposes of this application, it is sufficient to note that the orifice portion of nozzle 39 which feeds into the carburetor is closed by an oscillating valve 40 which is normally urged to a closed position by a spring means 41 whose tension can be varied in accordance with varying requirements of fuel flow. In order to achieve proper fuel atomization, it is desired to have valve 40 oscillate between open and closed positions relative to nozzle 39., such oscillations being obtainable ifa pulsating fuel flow is supplied to nozzle 39. In order for the valve 40 to be able to oscillate smoothly and in order for the flow through nozzle 39 to be controllable solely by spring 41, it is necessary that the pulses be distinct and be of a constant magnitude.

In operation, therefore, liquid fuel is pumped from its storage tank and into passage 7 which leads into cylindrical space 2 in correspondence to the annular recess 5. This fuel then passes from recess 5 axially through passages 4 to the inlet side of non-return valve body 9.

While piston 3 is being moved along its compression stroke (upwards in the figure) by means 33, 34, etc., valve body 9 is forced to closed position against seat 12 by the liquid pressure in chamber 15. However, when piston 3 is accelerated in a downward direction (its suction stroke) from its top dead center" position under the action of spring 35, the valve body 9 lags behind said piston to an extent equal to the clearance between hole 13 and pin 14, this clearance being in the order of only a few tenths of a millimeter (e.g., 0.2 mm for a stroke of 0.8 mm) but being sufficient for fuel to pass along seat 12 from passages 4 and into compression chamber l5. When piston 3 reaches its bottom dead center position, valve body 9 still has a downwardly directed momentum and thereby closes against seat 12 whereupon the piston 3 commences its upward compression stroke and valve body 9 then is maintained tightly against seat 12 by the increasing pressure in chamber 15. The pressure in chamber 15, passage 16, and nozzle 39 then builds up to a point at which. valve 40 then opens.

In order to obtain sufficiently distinct pulses and small pressure losses, it is necessary that the inoperative or dead space in the pulse chamber 15 be reduced as much as possible, and that the discharge line to nozzle 39 be as short as possible. The first condition is fulfilled by having the radial width of the pulse chamber 15 equal to the width of the cylinder and its axial length only slightly larger than the axial length of the piston stroke, whereby the piston comes very close to the upper end of space 2 when it is at its top dead center position. In fact, according to an actual embodiment, it is possible to reduce this axial height of chamber 15 to almost zero by adjusting shoe 31 so that the piston is in contact with the upper end of space 2 when said piston is at said top dead center position.

The length of the piston stroke is varied in dependence on the desired rate of fuel flow and, during operation, is automatically adjusted in accordance with the back pressure which develops in nozzle 39, which back pressure depends upon the tension in spring 41.

The automatic adjustment of the length of stroke in correspondence to the back pressure in the pulse chamber is effected through the resilient coupling 20. When the back pressure increases the cup spring 22 is compressed to a corresponding degree, depending upon its tension and characteristics. Its tension can, for this purpose, be set by means of the nut 26. The length of the piston rod 17, and thus the compression clearance between the piston and the pulse chamber wall, can be adjusted by means of the nut 32.

The major portion of the liquid flowing into port 6 continues through annular passage 5 and out passage 38 and recirculates to the storage tankwhere entrained gas bubbles may be liberated therefrom. This continuous recirculation of liquid prevents any gas bubbles from being entrained within the liquid which flows through passages 4 into pulse chamber 15, since, despite all precautions, some gas bubbles will unavoidably develop within port 6 as the result of the rapid reciprocation of the piston, leakage, etc. It is to be noted, in this regard, that the recirculation of liquid according to the invention occurs on the suction side of the piston and not on the pressure side, that is: from passage 16, whereby the recirculation according to the invention does not entail any power loss insofar as the pulse generator itself is concerned.

The embodiment of H6. 2 is analogous to that of FIG. 1 in many respects and, therefore, various heretofore presented explanations relative to the first embodiment will not necessarily be repeated relative to H6. 2 in instances in which the same basic principles of operation obviously apply. it will be noted that many elements in H6. 2 which are analogous to elements in FIG. l have been identified by the same numbers as in FIG. 1 but in the hundred series.

In FIG. 2, housing llllil defines a piston space M2 in which a piston means including pulse piston i103 and valve body 109 is reciprocably displaceably arranged. A flexible diaphragm MM- is clamped in a sealing manner respectively to the piston 103 and to the housing ltll so to define a continuous radial seal therebetween to thereby sealingly separate the compression chamber 1115 from the remainder of space W2, said diaphragm in the meantime permitting axial displacement of the piston relative to the housing. Housing Mil includes a feed port Ml? leading into an inlet chamber W6 which in turn connects with axially extending passageway means i105 along valve body MW.

Valve body W9 includes a perforated plate 111 at one end thereof and a valve diskv portion lllil at the other end thereof. Said valve body 1109 is axially reciprocable relative to the housing ll llll between respective opposite axial positions thereof defined respectively by abutment of plate ill with housing shoulder i113 and seating of valve disk portion Md against housing valve seat H2. The extent of such reciprocation, that is the length of stroke, is determined by the extent to which the distance between plate llllll and disk llltl exceeds the distance between shoulder M3 and seat lll2.

Piston W33 includes an axially extending pin member 108 received within a bore lllld in valve body 1109, a friction ring i211 mounted on member 108 serving to frictionally connect pin W8 and body E09 together.

it is seen, therefore, that valve body N99 comprises a non-return valve acting between compression chamber 11115 and passageways 105, permitting flow of liquid fuel from chamber 106 and along passageways W5 and into chamber 1115 when disk portion ill) is spaced from seat 112 but preventing backflow from chamber 1115 into passageways MP5 when said disk portion lllti is seated against said seat.

An outlet passageway 1116 leads from pulse chamber 115 to externally of housing Mill to elements 139, Mil, and R41 which are the same as 39, 40., and M in FIG.

Piston 103 is provided with a piston rod means H7 which in turn is made up of two axially successive rod portions H8 and 119 which are axially coupled together by means of a resilient coupling means 120..

Coupling means 120 serves to resiliently interconnect the two separate members 1118 and 119 which constitute a piston rod means for piston 1103. in this regard, sleeve M8 is integral with the piston R03 and rod portion H9 is slidingly fitted within the bore of sleeve member H8. Rod portion 11119 comprises a pair of axially spaced apart abutment members 1123 and K2 8, the first of which is abutable against one end of said bore and the second of which, 124, serves to confine one end of a compression spring, such as cup spring means 122, whose other end abuts against a shoulder 125 of sleeve 1118. Compression spring 135 is mounted externally of housing Mil but is analogous to spring 35 in the first embodiment since it also is confined between housing shoulder H37 and shoulder 136 in the piston rod member 1118. This member M8 is axially slidably fitted within bore 130 in housing Mil.

in this second embodiment, the abutments shoulder 124 and the follower shoe H31 constitute one integral member threadedly adjustably fitted onto one end of rod portion 119..

The motor H34- and earn 133 are analogous to the corresponding elements 34 and 33 in FIG. 1 and function in the same manner relative to shoe 131 and the other elements of the pulse generator.

In the H6. 2 embodiment, the tension on spring 122 and the axial position of rod portion 11% relative to shoe 1131 are both variable integrally rather than separately as is the case in the FlG. l embodiment. Further, the FIG. 2 embodiment includes an equalizing means comprising a diaphragm 126 extending across one end of chamber res opposite to valve body 109, said diaphragm being biased towards said valve member by a coil spring 1128 compressed between the housing 101 and a thrust plate 127 which bears against one side of the diaphragm.

The pulse generator functions in the following manner. Fuel, e.g. petrol, is pumped from a fuel tank, in the direction of the arrow in H0. 2, through the inlet 107 to the inlet chamber we. The fuel then passes through perforations in the plate ill and along the passages to the non-return valve means 109,112. The piston means Hi3 is driven alternately along a pumping and suction stroke by means 133. During the suction stroke the piston carries along with it the valve body 109 by virtue of the friction provided therebetween by the friction ring l2l, until the plate illlll comes into contact with abutment M3. The length of stroke of the valve body 109 has been exaggerated in the drawing, and in actual fact only amounts to a fraction of the length of stroke of the piston W3, e.g., about 0.2 mm in a stroke length of about 0.8 mm. As the valve disk ill) leaves its seat 112, fuel passes into the pulse chamber and when the piston has reached the end of its suction stroke and returns along its pumping stroke, the valve 1M) closes rapidly by virtue of the pressure build up in chamber llllS. During its pumping or compression stroke, the piston MP3 forces out a jet of liquid fuel from the pulse chamber llllS through the outlet 116, as shown by the dotted arrow, to a means requiring same, in this instance the nozzle 139.

In order to obtain sufficiently distinct pulses and to keep pressure losses at a minimum, it is necessary that the inoperative or dead space in the pulse chamber 1 l5 be as small as possible, as already explained with reference to H6. l, and that the line 139 to the consumer means be as short as possible. The first condition is fulfilled by making the width of the pulse chamber 115 substantially equal to the width of the cylinder and its axial length only slightly greater than the length of the piston plus the stroke of the piston 103, whereby at the end of the pumping stroke the clearance between the end surface of the piston 1103 and the valve disk portion l M) is so small that the piston almost comes into contact with the disk. The length of stroke of the piston varies in dependence on the desired fuel flow, and is adjusted automatically according to the back pressure in the injector nozzle 139, which depends upon the tension of spring Mil, as has already been explained with reference to FIG. 1.

The clearance between the piston MP3 and the disk 110 can be adjusted by changing the axial disposition of shoe 1131 along rod member 119 analogously to the arrangement of FIG. 1.

In the FIG. 1 embodiment, the valve 9 is arranged in the pulse piston 3 and the drop in pressure occurring when the fuel is conveyed from the inlet chamber 6to the pulse chamber i is very slight. This results because the energy required to move the valve body 9 is transferred to said body from the piston partly through the pin 14 which connects them and partly through the valve seat 12 arranged in the piston. in this way the valve is positively controlled by the piston, as opposed to conventional non-return valves controlled by the pressure conditions in the liquid. In the H6. 2 embodiment, the same effect is obtained by transferring energy through the friction ring 121. Another important feature of the H6. 2 embodiment is that the described construction of the valve results in a relatively large valve opening occurring at Hi2 thus resolving the problems mentioned in the presented herein introduction, among them the drop in pressure experienced in known devices.

Recirculating line T38 functions in the same manner set forth with regard to passageway 38 in FIG. I.

The herein given details of a preferred embodiment are given by way of illustration only and are not intended to be limitative of the scope of the invention; it being understood that all modifications and substitutions of either an obvious nature or well within the purview of one skilled in the art are intended to be covered by the scope of this invention. For instance, the pulse piston in FIG. 2, instead of being connected with the housing by a diaphragm 104, may sealingly abut the walls of the cylinder chamber, as in the embodiment of FlG. l. Furthermore, instead of comprising a motor driven eccentric the drive means may be an electromagnet whose armature is connected with the piston rod and which is supplied with pulsating electric current, whose frequency thus determines the pulse frequency of the generator.

What is claimed is:

i. A hydraulic pulse generator comprising a housing having an internally axially extending space including a compression chamber, a piston member mounted in said space for reciprocation along a suction and compression stroke, respectively, respective inlet and discharge liquid passageways extending through said housing to said space, a non-return valve member arranged to respectively open and close said inlet passageway relative to said compression chamber portion in correspondence to said piston member respectively reciprocating along said suction and compression strokes, said piston and valve members including a bore in one member thereof and a pin on the other member thereof slidably fitted within said bore and being axially drivingly coupled together by a yieldable means comprising a friction contact means between said pin member and the walls of said bore.

2. The pulse generator of claim 1, wherein said discharge passageway leads from said compression chamber portion to outwardly of said housing, and further includes a recirculating passageway leading outwardly from said space whereby there is a continuous unobstructed communication from said inlet passageway through said space to said recirculating passageway.

3. The pulse generator of claim 1, wherein said valve member is mounted within said space for axially reciprocating therein relative to said piston member, said compression chamber portion being defined between said piston member and said valve member.

4. The pulse generator of claim 3, wherein said valve member is axially reciprocable between respective positions which correspond to said inlet passageway being respectively open and closed relative to said compression chamber portion, said piston member being axially displaceable relative to said valve member, respective facing surfaces on said piston and valve members defining opposite ends of said compression chamber portion.

5., The pulse generator of claim 3, including a flexible diaphragm means respectively sealingly clamped to said piston member and to said housing and extending radially across the peripheral clearance space therebetween so as to peripherally seal said piston member and housing against leakage therebetween while allowing axial reciprocation of said piston member relative to said housing.

6. The pulse generator of claim 3, wherein said space is defined at one end thereof by an axially flexible diaphragm and at an opposite end thereof by said piston member, said compression chamber portion being defined between said piston member and said valve member, a liquid holding chamber being defined between said diaphragm and said valve member, said inlet passageway leading into said chamber, and including a resilient means bearing against one side of said diaphragm to urge same axially towards said piston member.

7. The pulse generator of claim 3, wherein said hous' ing includes axially spaced apart abutment means in said space and said valve member includes corresponding abutment means spaced apart to a greater extent than the housing abutment means and respecitvely cooperating therewith to define the opposite limits of a reciprocatory stroke for said valve member.

8. The pulse generator of claim 7, wherein said abutment means on said valve member and said abutment means on said housing comprise a face and seating respectively.

9. The hydraulic pulse generator of claim 1, including an elongate piston rod means for reciprocably driving said piston member, said rod means comprising first and second axially separate elongate portions interconnected by an axially resilient deformable coupling means whereby said portions are axially displaceable relative to each other, said first elongate portion being rigid with said piston member.

10. The pulse generator of claim 9, wherein said coupling means comprises a compression spring means compressibly mounted between said first and second rod portions and includes a shoe stationarily mounted on said second rod portion adapted to be acted upon for driving said second portion in one axial direction and means to vary the position of said shoe along the axis of said second portion.

1 l. The pulse generator of claim it), including means to vary the tension in said spring means.

12. The pulse generator of claim 10, wherein said drive means is an electromagnet supplied with pulsating electric current and the armature of which is arranged to act upon the shoe, the frequency of the supply current thus determining the pulse frequency of the generator.

13. The pulse generator of claim 10, including a drive means for periodically driving said second portion in one axial direction, said drive means comprising a driving member acting upon said shoe.

14. The pulse generator of claim 13, wherein said drive means is a motor driven eccentric which abuts said shoe and the speed of which thus determines the pulse frequency of the generator.

15. The pulse generator of claim 10, including means said spring means tension. 

1. A hydraulic pulSe generator comprising a housing having an internally axially extending space including a compression chamber, a piston member mounted in said space for reciprocation along a suction and compression stroke, respectively, respective inlet and discharge liquid passageways extending through said housing to said space, a non-return valve member arranged to respectively open and close said inlet passageway relative to said compression chamber portion in correspondence to said piston member respectively reciprocating along said suction and compression strokes, said piston and valve members including a bore in one member thereof and a pin on the other member thereof slidably fitted within said bore and being axially drivingly coupled together by a yieldable means comprising a friction contact means between said pin member and the walls of said bore.
 2. The pulse generator of claim 1, wherein said discharge passageway leads from said compression chamber portion to outwardly of said housing, and further includes a recirculating passageway leading outwardly from said space whereby there is a continuous unobstructed communication from said inlet passageway through said space to said recirculating passageway.
 3. The pulse generator of claim 1, wherein said valve member is mounted within said space for axially reciprocating therein relative to said piston member, said compression chamber portion being defined between said piston member and said valve member.
 4. The pulse generator of claim 3, wherein said valve member is axially reciprocable between respective positions which correspond to said inlet passageway being respectively open and closed relative to said compression chamber portion, said piston member being axially displaceable relative to said valve member, respective facing surfaces on said piston and valve members defining opposite ends of said compression chamber portion.
 5. The pulse generator of claim 3, including a flexible diaphragm means respectively sealingly clamped to said piston member and to said housing and extending radially across the peripheral clearance space therebetween so as to peripherally seal said piston member and housing against leakage therebetween while allowing axial reciprocation of said piston member relative to said housing.
 6. The pulse generator of claim 3, wherein said space is defined at one end thereof by an axially flexible diaphragm and at an opposite end thereof by said piston member, said compression chamber portion being defined between said piston member and said valve member, a liquid holding chamber being defined between said diaphragm and said valve member, said inlet passageway leading into said chamber, and including a resilient means bearing against one side of said diaphragm to urge same axially towards said piston member.
 7. The pulse generator of claim 3, wherein said housing includes axially spaced apart abutment means in said space and said valve member includes corresponding abutment means spaced apart to a greater extent than the housing abutment means and respecitvely cooperating therewith to define the opposite limits of a reciprocatory stroke for said valve member.
 8. The pulse generator of claim 7, wherein said abutment means on said valve member and said abutment means on said housing comprise a face and seating respectively.
 9. The hydraulic pulse generator of claim 1, including an elongate piston rod means for reciprocably driving said piston member, said rod means comprising first and second axially separate elongate portions interconnected by an axially resilient deformable coupling means whereby said portions are axially displaceable relative to each other, said first elongate portion being rigid with said piston member.
 10. The pulse generator of claim 9, wherein said coupling means comprises a compression spring means compressibly mounted between said first and second rod portions and includes a shoe stationarily mounted on said second rod portion adapted to be acted upon for driving Said second portion in one axial direction and means to vary the position of said shoe along the axis of said second portion.
 11. The pulse generator of claim 10, including means to vary the tension in said spring means.
 12. The pulse generator of claim 10, wherein said drive means is an electromagnet supplied with pulsating electric current and the armature of which is arranged to act upon the shoe, the frequency of the supply current thus determining the pulse frequency of the generator.
 13. The pulse generator of claim 10, including a drive means for periodically driving said second portion in one axial direction, said drive means comprising a driving member acting upon said shoe.
 14. The pulse generator of claim 13, wherein said drive means is a motor driven eccentric which abuts said shoe and the speed of which thus determines the pulse frequency of the generator.
 15. The pulse generator of claim 10, including means to vary the tension in said compression spring means.
 16. The pulse generator of claim 15, wherein said shoe comprises part of a single member which is adjustably mounted on said second rod portion, said spring means being compressed between a part of said first rod portion and said single member, whereby axial adjustment of said single member along said second rod portion results in adjustment of said shoe along said second rod portion and simultaneous adjustment of said spring means tension. 